Fatigue crack propagation in 2.25 Cr-1.0Mo steel weldments in air and hydrogen

Fatigue crack growth (FCG) behaviour of 2.25Cr-1.0Mo steel plate and welds tested in air and gaseous hydrogen was investigated. Microstructural observ ations indicated that both the weld metal GYM) and heat-affected zone (HAZ) were composed of mainly bainitic microstructure. A ferrite matrix with uni formly dispersed carbides was found in the annealed base metal. The change in microstructures of WM and HAZ between the as-welded and tempered welds w as limited. The as-welded WM had the lowest impact toughness among the spec imens, however, the impact toughness was improved after postweld heat treat ment at 700 degreesC/1 h. Regardless of the testing environments, the as-we lded WM exhibited a higher resistance to crack growth than the steel plate as the crack propagated transversely to the welding direction, especially i n the low DeltaK range. However, higher fatigue crack growth rates in the a s-welded HAZ than those of the steel plate were found. As the crack grew al ong the fine-grained heat-affected zone in the as-welded condition, signifi cant retardation of crack growth as compared with that of the steel plate w as obtained when tested in air or gaseous hydrogen. In various regions of t he weld, the FCG behaviour of tempered welds was similar to that of the ste el plate in air. The results also indicated that the tempered bainitic micr ostructure present in WM or HAZ was more resistant to unstable crack growth than the annealed steel plate tested in gaseous hydrogen. Environmentally induced acceleration of FCG in various regions of the weld was usually exhi bited a change of fracture features. (C) 2001 Elsevier Science B.V. All rig hts reserved.